You can convert a SQLite database to a better performing MySQL, MariaDB or
PostgreSQL database with the Nextcloud command line tool. SQLite is good for
testing and simple single-user Nextcloud servers, but it does not scale for
multiple-user production users.

First set up the new database, here called “new_db_name”.
In Nextcloud root folder call

phpoccdb:convert-type[options]typeusernamehostnamedatabase

The Options

--port="3306" the database port (optional)

--password="mysql_user_password" password for the new database. If omitted the tool will ask you (optional)

--clear-schema clear schema (optional)

--all-apps by default, tables for enabled apps are converted, use to convert also tables of deactivated apps (optional)

Note: The converter searches for apps in your configured app folders and uses
the schema definitions in the apps to create the new table. So tables of removed
apps will not be converted even with option --all-apps

For example

phpoccdb:convert-type--all-appsmysqloc_mysql_user127.0.0.1new_db_name

To successfully proceed with the conversion, you must type yes when prompted
with the question Continuewiththeconversion?

On success the converter will automatically configure the new database in your
Nextcloud config config.php.